Feasibility of thermo-sensitive P(NIPAM-MBA) hydrogels as novel stripping agents for osmotic membrane distillation

Abstract

As thermo-sensitive hydrogels could absorb water quickly and squeeze the water out by their insoluble cross-linked networks under thermal stimulus, a novel osmotic membrane distillation (OMD) process was proposed for the first time. In the OMD process of this study, water vapor diffused across the membrane under the swelling pressure of thermo-sensitive hydrogels. Poly-N-isopropylacrylamide (PNIPAM) hydrogels with 0.02%, 0.1% and 0.5% N,N′-methylenebisacrylamide (MBA) were synthesized and incorporated into the OMD system. The hydrogel properties were characterized by UV–vis spectrophotometer, FTIR and SEM. The effects of MBA content, membrane characteristics, operating temperature on OMD flux were investigated. The initial fluxes for the 0.02%, 0.1% and 0.5% MBA hydrogel-OMD systems were 1.68, 1.09 and 0.76 kg/m2·h, respectively. And the hydrogels transformed to dehydrated state after 150 min of water absorption. The fastest water absorption rates for PNIPAM hydrogels with 0.02%, 0.1% and 0.5% MBA were obtained at the initial swelling ratios of 16, 14 and 12 folds, respectively. Besides, the optimal feed temperatures for the maximum flux were 40 °C, 50 °C and 50 °C for hydrogels with 0.02%, 0.1% and 0.5% MBA, respectively. The membranes with higher porosity and lower thickness demonstrated higher fluxes. According to our results, the swelling pressure of hydrogel acted as a driving force of the hybrid hydrogel-OMD system, and it was greatly affected by the hydrogel structure. Typically, a hydrogel with the balanced water absorbing and dewatering behaviors and a membrane with good mass transfer performance were two essential prerequisites for this process.